Distributed antenna system for MIMO technologies

ABSTRACT

A method and system for supporting M1MO technologies which can require the transport of multiple spatial streams on a traditional Distributed Antenna System (DAS). According to the invention, at one end of the DAS, each spatial stream is shifted in frequency to a pre-assigned band (such as a band at a frequency lower than the native frequency) that does not overlap the band assigned to other spatial streams (or the band of any other services being carried by the DAS). Each of the spatial streams can be combined and transmitted as a combined signal over a common coaxial cable. At the other “end” of the DAS, the different streams are shifted back to their original (overlapping) frequencies but retain their individual “identities” by being radiated through physically separate antenna elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 13/598,078 filed on Aug. 29, 2012 and entitled“Distributed Antenna System for MIMO Technologies,” which is acontinuation of U.S. patent application Ser. No. 11/958,062 filed Dec.17, 2007, which claims the benefit of Provisional Application No.60/870,739 filed on Dec. 19, 2006, all of which are hereby incorporatedby reference in their entireties.

RELATED APPLICATION

The present application is related to U.S. patent application Ser. No.12/026,557 filed on Feb. 6, 2008 and entitled “MIMO-Adapted DistributedAntenna System.”

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

REFERENCE TO MICROFICHE APPENDIX

Not Applicable.

BACKGROUND Technical Field of the Invention

The present invention is directed to Distributed Antenna Systems andmore particularly, to methods and systems for transmitting multiplesignals or spatial streams over the same RF frequencies using aDistributed Antenna System (“DAS”).

The present invention is directed to a DAS intended to support wirelessservices employing MIMO technologies, such as a WiMax network.Traditionally, a base station connected to a DAS transmits a singlesignal (one or more RF carriers) within a frequency band. In the case ofa MIMO-enabled base station, multiple signals, often referred to asspatial streams, are transmitted on the same RF frequencies. In orderfor a DAS to adequately support the distribution of this service, itneeds to carry the multiple spatial streams to each radiating point, andat each radiating point radiate (and receive) the different streams onseparate antenna elements.

One challenge for a traditional DAS architecture in addressing theserequirements is that a traditional DAS carries signals at their nativeRF frequency. Therefore carrying multiple signals at the same frequency(namely the multiple spatial streams) may require the deployment ofparallel systems.

SUMMARY OF THE INVENTION

In referring to the signal flows in DAS systems, the term Downlinksignal refers to the signal being transmitted by the source transmitter(e.g. cellular base station) through an antenna to the terminals and theterm Uplink signal refers to the signals being transmitted by theterminals which are received by an antenna and flow to the sourcereceiver. Many wireless services have both an uplink and a downlink, butsome have only a downlink (e.g. a mobile video broadcast service) oronly an uplink (e.g. certain types of medical telemetry).

In accordance with the invention, multiple spatial streams aretransported on a traditional DAS architecture whereby, at the input end,each spatial stream is shifted in frequency to a pre-assigned band (suchas a band at a frequency lower than the native frequency) that does notoverlap the band assigned to other spatial stream (or the band of anyother services being carried by the DAS). At the other “end” of the DAS,the different streams are shifted back to their original (overlapping)frequencies but retain their individual “identities” by being radiatedthrough physically separate antenna elements. In one embodiment,frequency shifting can be implemented using frequency mixers.

Most wireless services of interest in this context are bi-directional,meaning they have both a Downlink (signals transmitted from Base stationto terminals) and an Uplink (signal transmitted from terminal to Basestation). Some wireless technologies operate in FDD (Frequency divisionduplexing) mode, meaning Downlink (DL) and Uplink (UL) operatesimultaneously on different frequencies, while others operate in TDD(Time division duplexing) mode, meaning DL and UL alternate in timeusing the same frequency bands.

The cabling technologies used in a DAS can differ in the way theytransfer DL and UL on the same medium (e.g., cable or fiber). Fiberlinks can use a separate fiber strand (or wavelength in WDM systems) forUL and DL. Therefore, Fiber links can easily support both FDD and TDDmodes.

Coax links usually use a single cable for both DL and UL. For FDDservices, this does not present a problem since the DL and UL signalscan use different frequencies. For TDD services, two differentembodiments can be used. In one embodiment, a separate frequency for DLand UL can be used (meaning one or both of the DL and UL need to beshifted from their native, overlapping frequencies to non-overlappingfrequencies). In an alternative embodiment, a switching mechanism can beused to alternate the DL and the UL transmission on the same frequency.This embodiment has the advantage of using less spectrum resources,allowing other services (at other frequencies) to run on the same cable.

These and other capabilities of the invention, along with the inventionitself, will be more fully understood after a review of the followingfigures, detailed description, and claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of an embodiment of a distributed antennasystem according to the invention;

FIG. 2 is a block diagram of an alternate embodiment of a distributedantenna system according to the invention; and

FIG. 3 is block diagram of an alternative embodiment of a distributedantenna system according to the invention.

DESCRIPTION OF THE INVENTION

In accordance with the invention, a method and system can be implementedin a DAS architecture which uses both fiber links and coax links, for aMIMO service using 2 or more spatial streams and operating in TDD mode.Other configurations, such as those supporting 3 or more specialstreams, would require simple variations on the scheme presented below.

FIG. 1 shows an example of a DAS 100 in accordance with the invention.The DAS can include a Radio Interface Unit (RIU) 110, a Base Unit (BU)120, a Remote Unit (RU) 130 and an Antenna Unit (AU) 150.

The RIU 110 provides the interface to the Base station (BTS, not shown).In this embodiment, the RIU has two DL connections from the BTS and twoUL connections to the BTS, however a single DL/UL connection or morethan two DL and UL connections can be carried by the system. The RIU 110can include a mixer 112 on each DL connection and a mixer 112 on each ULconnection. The RIU 110 can implement the frequency shifting(“down-converting”) for the multiple DL spatial stream signals, mappingeach to a different non-overlapping frequency band. For example the DLsignals can be down-converted from the WiMAX 2.5 GHz-2.7 GHz frequencybands to the 100 MHz-300 MHz frequency band or the 320 MHz-520 MHzfrequency band. It implements the opposite for the UL signals. Themixers 112 can change the signal frequency on each DL connection to adifferent non-overlapping frequency band so that all the signals can becarried on the same cable without interference. The duplexer 114 acombines the DL connections (which use different frequency bands) onto acommon cable and can output the signals to the BU 120.

Similarly, the UL signals received from the BU 120 can be input into ade-duplexer 114 b, which separates the UL into separate connections.Each of UL connections can be input to a mixer 112 and converted back totheir original or native frequency bands for transmission back to theBTS. For example, the UL signals can be up-converted from the 100MHz-300 MHz frequency band or the 320 MHz-520 MHz frequency band to theWiMAX 2.5 GHz-2.7 GHz frequency. In an alternative embodiment the samefrequencies can be shared for DL and UL and the same circuits and mixerscan be used for both DL & UL, alternating in time. In accordance withthe invention, where the same frequencies are shared by the DL and UL,the same circuits and mixers can be used for both the DL and UL signalpaths, alternating in time using, for example, time divisionmultiplexing.

The BU 120 can convert the DL RF signal to an optical signal and splitthat signal into multiple optical links 122 which can be connected tomultiple Remote Units RUs 150. The BU 120 implements the opposite for ULsignals. The BU 120 allows the signals to be distributed, for example,to multiple buildings of campus wide network or multiple floors of abuilding. The BU 120 can be a dual point to multi-point device thatconverts an input RF DL signal in to multiple optical output signals,for example to transmit the signals over a fiber-optic link 122 andreceives multiple optical input signals and combines them onto a singleRF UL signal. One example of a BU 120, is a Mobile Access Base Unitabove from MobileAccess Networks, Inc., of Vienna, Va.

The RIU 110 and BU 120 can be co-located and, optionally, can becombined into a single physical element or component. Where the RIU 110and the Bu 120 are co-located, coaxial cable or twisted pair copper wirecan be used to interconnect the units.

The RUs 130 can be located in wiring closets in different areas (e.g.floors) of a building. The RU 130 can include a media convertingcomponent 132, 134 for converting optical signals to electronic signals(DL connection) and electronic signals to optical signals (ULconnection), amplifiers 136 a, 136 b for amplifying the signals asnecessary, a time division duplexing (TDD) switching mechanism 137 forcombining the DL and UL signals on a common transmission medium, and amultiplexer 138 for splitting the signal for transmission to multipleantennae and receiving signals from multiple antennae. For the DLconnection, the RU 130 can transform the signals from optical to RF, beprocessed by the TDD switching mechanism 137, and using the multiplexer138, split the signals onto multiple coaxial cables 140 going tomultiple Antenna Units 150. The RU 130 implements the opposite for ULsignals. In addition the RU can provide powering over the coax cables tothe antenna units.

On the DL connection, the RU 130 can include a photo diode based system132 for converting the optical signal to an RF signal. An amplifier 136a can be provided to adjust the amplitude of the signal before it isinput into a time division duplexing (TDD) switch 137. The TDD switch137 can be connected to a multiplexer 138 which can connect the DLconnection to multiple Antenna Units AU 150 over a cable 140, such as acoaxial cable.

On the UL connection, the RU 130 receives RF signals from one or moreAils 150 and inputs each signal into multiplexer 138 which multiplexesthe UL signals onto a single connection. The single UL connection can befed into the TDD switch 137. The TDD switch 137 separates the ULconnection from the DL connection and converts the UL signal to anoptical signal. An amplifier 136 b can be provided to adjust theamplitude of the signal before transmission to the BU 120. The RU 130can include a laser based optical system 134 for converting theelectrical signals to optical signals.

The Antenna Units (AU) 150 can be located in the ceilings of thebuilding. For the DL, the AU 150 implements the TDD mechanism 152separating the DL and UL signals (opposite the RU 130), up-converts thetwo or more spatial channels to their native frequencies and transmitseach on a dedicated antenna element, with appropriate amplification. Forthe UL connection, the AU 150 implements the opposite for UL signals.The UL signals received from the antenna elements 164A, 166A areamplified 162 as necessary and then down-converted by mixers 158 fromtheir native frequencies to a non-overlapping intermediate frequency andcombined onto a single line by duplexer 156 b for transmission back tothe RU 130.

The AU 150 can include a TDD switch mechanism 152 for duplexing anddeduplexing (combining and separating) the UL connections and the DLconnections, an amplifier for the DL connections 154 a and the ULconnections 154 b, a deduplexer 156 a for recovering the two DLconnections, a duplexer 156 b for combining the two UL connections, amixer 158 for each DL connection for restoring the RF frequency of thesignal for transmission to the antenna 164A, a mixer 158 for each ULconnection for converting the RF frequency of each UL connection todifferent, non-overlapping frequency bands, amplifiers 162 for each ofthe DL and UL connection, a TDD switching mechanism 164 for channel 1which connects the RF signal to antenna 164A and a TDD switchingmechanism for channel 2 which connects the RF signal to antenna 166A.

For the DL, the AU 150 implements the opposite of the RU 130 in that itde-duplexes the signal into two or more spatial stream and up-convertsthe two or more spatial streams to the native frequency for transmissionon a dedicated antenna element, with the appropriate amplification. Forthe UL, the AU 150 down-converts the two or more spatial streams to alower frequency band and duplexes them onto a single cable fortransmission to the RU 130.

When the frequencies used for transport through the DAS (the“down-converted” signals) are relatively low, it is possible to use lowcost cabling such as Multi-mode fiber and CATV-grade coax (e.g. RG-11 orRG-6). For example, the down-converted signals can be in the 100 MHz-300MHz and 320 MHz-520 MHz frequency bands.

As shown in FIG. 2, the present invention can also be used to combineother services, such as non-MIMO services, on the same system, with thesame cabling infrastructure. Additional MIMO bands can be handled in thesame way, and they would be transported using additional non-overlappingfrequency bands with respect to the frequency bands used for the firstMIMO service. Non-MIMO bands can be transported at their nativefrequency and amplified at the RU, using passive antenna elements toradiate them at the AU.

In an embodiment similar to FIG. 1, FIG. 2 shows an embodiment of thepresent invention combined with other services. The DAS 200 includes aRadio Interface Unit (RIU) 210, a Base Unit (BU) 220, a Multiband RemoteUnit (RU) 230 and an Antenna Unit (AU) 250.

The RIU 210 can include two or more spatial stream inputs from BTS (notshown) and any number of other services, for example, Service 1, Service2, and Service 3. As described above with regard to FIG. 1, mixers 212can be used to down-convert the DL connection and up-convert the ULconnection, and a duplexer/de-duplexer 214 can be use can be used tocombine the DL streams and separate the UL streams. The RIU 210 sendsthe DL signals to the BU 220 and receives the UL signals from the BU220.

The other services can include any other service that uses frequencybands that do not interfere with the frequency bands already used by thesystem. In one embodiment of the invention, the spatial streams onChannel 1 and Channel 2 provide WiMAX network services in the 2.5-2.7GHz frequency band and the other services can include, for example, CDMAbased services (e.g. in the 1.9 GHz PCS band) and iDEN based services(e.g. in the 800 MHz and 900 MHz bands).

The BU 220 can be same as described above and shown in FIG. 1. The BU220 can be any device that converts the DL RF signal to an opticalsignal and splits the signal to feed multiple optical links and combinesthe UL optical signals received over multiple optical links and convertsthe UL optical signals into RF signals.

In accordance with the embodiment shown in FIG. 2, the Multiband RU 230receives the DL optical signals from the BU 220 and sends UL opticalsignals to the BU 220. The processing block 236 can include thecomponents of FIG. 1, including the photo diode based system forconverting the DL optical signals back to RF signals and the laser basedsystem for converting the UL RF signals to optical signals andamplifiers for adjusting the signal amplitude as necessary. Theprocessing block 236 can also include duplexer/de-duplexer system forcombining the DL RF signals with the signals for the other services andseparating the UL RF signals from the signals for other services. Theprocessing block 236 can also include a multiplexer for splitting thecombined DL signal to be transmitted to multiple antenna units 250 andfor combining the individual UL signals received from the multipleantenna units 250.

The AU 250 of FIG. 2 is similar to the AU 150 of FIG. 1, in that itincludes a TDD switching system 252, amplifiers 254 a and 254 b,de-duplexer 256 a, duplexer 256 b, mixers 258, amplifiers 262, TDDswitching system 264, TDD switching system 266, antenna 264 a andantenna 266 a. In addition, AU 250 includes duplexer/de-duplexer 268which separates the signals for the other services from DL RF signal andfeeds the signals for the other services to passive antenna 270 and thespatial streams to TDD switching system 252. For the UL signals, theduplexer/de-duplexer 268 combines the signals for the other serviceswith the spatial streams in order to send them to the Multiband RU 230.

In cases where significant capacity is required in a facility covered bya DAS, multiple base-stations (or multiple sectors on a single basestation) can be used to “feed” the DAS, where each segment of the DAScan be associated with one of the base stations/sectors. In order toprovide additional flexibility in assigning capacity to areas in thefacility, it is desirable to be able to independently associate each AUwith any one of the base stations/sectors.

In accordance with one embodiment of the invention, the RIU can havemultiple, separate interfaces for each base station/sector (2 spatialstreams from each in the 2-way MIMO example discussed above). The RIUcan map each pair of signals from each base station/sector to adifferent pair of bands, non-overlapping with the bands assigned toother base stations/sectors. The BU and RU can retain the samefunctionality as above. The AU can have the ability using software toselect the specific sector to use, based on tuning to the respectivefrequency bands.

However, one of the disadvantages of the approach described in theprevious paragraph is that multiple blocks of spectrum are required onthe link between the RU 130,230 and the AU 150,250 in order to supportmultiple sectors. This reduces the amount of spectrum available tosupport other services.

As shown in FIG. 3, in accordance with an alternative embodiment of theinvention, the system can maintain the same flexibility of associationof sectors to antennas and the functionality of the RIU is as describedabove (mapping each sector to a different frequency band). The RU 330can map all sectors to the same frequency band and use a switch 335 toselect the sector to be associated with each of its ports and each portbeing connected over a separate coax cable to a specific AU 350. In thisembodiment, the amount of spectrum consumed on the coax under thisscheme is the amount required to support a single sector, regardless ofthe number of sectors supported in the full system.

The embodiment of FIG. 3 is similar to FIGS. 1 and 2 above. The RIU 310can be connected to one or more BTS units (not shown). The RIU 310 caninclude mixers 312 and duplexer/de-duplexers 314 and be coupled to theBU 320 over a DL connection and an UL connection. The BU 320 can be thesame as BU 120 and BU 220 as describe above. Further, each antenna unitAU 350 can be the same as AU 150 or AU 250 as described above.

The RU 330 can be similar to RU 130 and RU 230, and include a photodiode based system 332 for converting the DL optical signals to RFsignal and a laser based system 334 for converting the UL RF signals tooptical signals, along with amplifiers 336 a, 336 b to for adjusting thesignal as needed.

For the DL spatial streams, the RU 330 includes a switch 335 whichselectively connects a particular DL spatial stream to one of set of TDDswitching systems 337 which is associated with a particular sector anduses multiplexer 338 to connect each sector to one or more antenna unitsAU 350. Each TDD switching system 337 can include a DL mixer forconverting the DL spatial stream to a common frequency band and an ULmixer for converting the UL spatial stream from the common frequencyband to the initial received frequency band. Each AU 350 can beconfigured to communicate using the common frequency band. The commonfrequency band can be selected based on environmental conditions and thedistances of the runs of cable 340 for the system. The common frequencycan be the same as the most common frequency used the RIU for convertingthe spatial streams, so no conversion is required for some signals (themost common) thus reducing the power requirements and potential forsignal distortion on the most common signals.

Other embodiments are within the scope and spirit of the invention. Forexample, due to the nature of software, functions described above can beimplemented using software, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations.

Further, while the description above refers to the invention, thedescription may include more than one invention.

We claim:
 1. An antenna unit for distributing downlink MIMO signals for at least one MIMO service in distributed antenna system (DAS) received over a single coaxial cable coupled to a single coaxial cable port, comprising: a single coaxial cable port coupled to a single coaxial cable, the single coaxial cable port configured to receive non-overlapping frequency MIMO downlink signals of at least one MIMO service over the single coaxial cable; at least two antenna unit downlink frequency shifters configured to frequency shift the non-overlapping frequency MIMO downlink signals to native frequency MIMO downlink signals; and at least two downlink output ports configured to receive and couple the native frequency MIMO downlink signals to at least two MIMO antennas, the at least two MIMO antennas configured to transmit the native frequency MIMO downlink signals, wherein the at least two antenna unit downlink frequency shifters are each configured to frequency shift the non-overlapping frequency MIMO downlink signals in a frequency range of 320 MHz to 520 MHz to native frequency MIMO downlink signals.
 2. The antenna unit of claim 1, wherein the at least two antenna unit downlink frequency shifters are comprised of: a first antenna unit downlink frequency shifter configured to frequency shift a first non-overlapping frequency MIMO downlink signal among the non-overlapping frequency MIMO downlink signals to a first native frequency MIMO downlink signal; and a second antenna unit downlink frequency shifter configured to frequency shift a second non-overlapping frequency MIMO downlink signal among the non-overlapping frequency MIMO downlink signals to a second native frequency MIMO downlink signal.
 3. The antenna unit of claim 2, further comprising an antenna unit downlink splitter configured to split the non-overlapping frequency MIMO downlink signals into the first non-overlapping frequency MIMO downlink signal and the second non-overlapping frequency MIMO downlink signal.
 4. The antenna unit of claim 2, further configured to communicate the first native frequency MIMO downlink signal to a first MIMO antenna among the at least two MIMO antennas, and communicate the second native frequency MIMO downlink signal to a second MIMO antenna among the at least two MIMO antennas.
 5. A method of distributing downlink MIMO signals for at least one MIMO service received by an antenna unit in distributed antenna system (DAS) over a single coaxial cable port coupled to a single coaxial cable, comprising: receiving non-overlapping frequency MIMO downlink signals of at least one MIMO service from a single coaxial cable port coupled to a single coaxial cable; frequency shifting the non-overlapping frequency MIMO downlink signals in at least two downlink frequency shifters to native frequency MIMO downlink signals; and transmitting the native frequency MIMO downlink signals over at least two MIMO antennas receiving the native frequency MIMO downlink signals, wherein frequency shifting the non-overlapping frequency MIMO downlink signals further comprises frequency shifting the non-overlapping frequency MIMO downlink signals in a frequency range of 320 MHz to 520 MHz to the native frequency MIMO downlink signals. 